1. Field of the Invention
This invention relates to a block structure in a tread portion of a pneumatic tire capable of improving steering stability on a wet road surface and resistance to hydroplaning while maintaining a wear resistance of a block.
2. Description of Related Art
In order to mainly improve the steering stability on a wet road surface, it has widely been practiced to form various kinds of sipes in a block constituting a ground contact face of a tread portion in a pneumatic tire.
However, when the sipe is formed in the block constituting the ground contact face to improve the steering stability on the wet road surface, the rigidity of the block lowers and hence there is caused a problem that the wear resistance is degraded due to the lowering of the rigidity.
It is, therefore, an object of the invention to solve the above problem of the conventional technique and to provide a block structure in a tread portion of a pneumatic tire improving steering stability on a wet road surface and resistance to hydroplaning while maintaining a wear resistance of a block.
According to the invention, there is the provision of a block structure in a ground contact face of a tread portion of a pneumatic tire comprising blocks defined by circumferential grooves extending in a circumferential direction and lateral grooves extending in a widthwise direction at the ground contact face and a sipe formed in each surface of the blocks in a block row at least located near to a center of the tread and extending in the widthwise direction and opening at least one end thereof to the respective circumferential groove, in which the sipe is opened through a width-widened part to the circumferential groove and a depth of the width-widened part is made shallower than a depth of a deepest part of the sipe.
The wear resistance of the block is degraded due to the fact that the rigidity of the block is lowered by the sipe. Such a lowering of the rigidity in the block is largely affected by a depth of a portion of the sipe that is opened to the circumferential groove. In the invention, therefore, the lowering of the block rigidity is controlled by opening the sipe to the circumferential groove through the width-widened part having a depth shallower than the depth of the sipe, whereby the degradation of the wear resistance in the block can effectively be prevented.
On the other hand, the drainage performance is degraded due to the fact that the depth of the width-widened part is made shallower than the depth of the sipe. In the invention, however, such a degradation is superfluously offset by making the width of the width-widened part wider than the width of the sipe.
According to the above block structure, therefore, the degradation of the wear resistance in the block can be prevented under the condition that the width-widened part opening to the circumferential groove is made shallower than the depth of the sipe, while the steering stability on the wet road surface and resistance to hydroplaning can advantageously be improved by making the width of the width-widened part wider than that of the sipe.
In a preferable embodiment of the invention, the depth of the width-widened part is 25-50% of a depth of a deepest part of the sipe, and further the depth of the width-widened part is 20-30% of a height of the block.
In the former case, when the depth is less than 25%, there is a fear that the sufficient drainage performance is not obtained, while when it exceeds 50%, the rigidity of the block is too low and hence the wear resistance lowers and the steering stability on the wet road surface is apt to be degraded. These facts are true even in the latter case.
In another preferable embodiment of the invention, the width of the width-widened part is 3-5 times a width of the sipe being, for example, about 0.7-1.2 mm, and a length of the width-widened part in the widthwise direction of the block is 10-30% of the width of the block.
In any case, when the value is less than the lower limit, there is a fear that the sufficient drainage performance can not be ensured, while when the value exceeds the upper limit, the ground contact area is too small and the steering stability on the wet road surface is apt to be degraded.
In the other preferable embodiment of the invention, the depth of the sipe is made deepest at a central part in its extending direction to prevent the lowering of the block rigidity. In this case, the depth of the deepest part is 2-3 times a depth of a shallowest part in order to ensure the sufficient drainage performance and prevent the lowering of the block rigidity for ensuring the steering stability on the wet road surface.